This presentation gives an idea about antimicrobial susceptibility, various methods to determine MIC, Action spectrum of antibiotics, etc.

2. Modern era in antibiotics started with
Alexander Fleming

4. • Antimicrobial agent:- It is the substance
which kills micro-organisms or inhibit their
growth.
They may be bacteriostatic or
bacteriocidal
• Antimicrobial susceptibility:-It is the
susceptibility of organisms towards
antimicrobial agents.

5. NARROW SPECTRUM
They are effective only
against a limited variety of
microorganims .
Eg.Vancomycin
Erthromycin
polymyxin
Bacitracin
BROAD SPECTRUM
They are effective against
wide variety of pathogenic
organisms.
Eg.Penicillin
Streptomycin
Sulfonamides
Gentamycin

6. • It guides the physician to choose the best
antimicrobial agent for treatment.

7. • It controls the use of inappropriate
antimicrobial agent in clinical practice.
• It shows the changing trends in local isolates.
• It is used to evaluate in vitro activity of new
agents.

8. • It determines if the organism has developed a
resistance mechanism or not.

9. • MIC:- It is the minimum amount of
antimicrobial agent that will inhibit visible
growth of an organism after over night
incubation under specific test condition.
• It helps the physician to choose appropriate
dosage of antimicrobial agent for treatment of
infection(usually 10 times of MIC is used)

10. 1) Tube dilution method
(¡) Broth macrodilution
(¡¡) Broth microdilution
2)Disk diffusion method
(¡) Kirby-bauer method
(¡¡) Multi tiped ring
(¡¡¡) Hi comb
3)Epsilometer test (E-
test)
4)Automated systems
5) Mechanism specific
tests
6) Genotypic methods

11. Broth Macro-dilution
No
AMA

13. Broth micro-dilution
• It is same as macro-dilution
• It uses about 0.05-0.1ml of broth
• Micro-
dilution
panel
contains
98 reagent
wells

16. Advantage
• Antibiotic of larger M.W. can also be
tested(because no diffusion is required).
• Contamination not occurs easily.
Disadvantage
• Time consuming.
• MIC end point not easily read.
• At a time one organism and one
antimicrobial agent can be tested.

18. Kirby-Bauer Method
• Principle:- As the antibiotic impregnated disks
are placed on agar they absorb moisture and
release antibiotics into agar.
As the distance from disks increases, the conc.
Of antibiotic decreases logarithmically.

19. Requirements
1.Mueller-hinton agar plate.(medium contains
beef extract, peptone and starch)
• It gives satisfactory growth of most
nonfastidious pathogens.
• It is low in sulfonamides & tetracycline
inihibitors.
• A large data has been collected by performing
susceptibility test using this medium.

20. 2. Test organism/ inoculum
3.Sterile disk:- commercially prepared disk are
6mm in diameter and are loaded with particular
concentration of antimicrobial agent.
4. Turbidity standard
5. Forceps & alcohol

21. Preparation of turbidity standards
BaCl2 + H2SO4 ->BaSO4 (ppts out)
Mc Farland tubes
0.5 Mc. Farland =
1.15x108 CFU/ml

22. Select the colony

26. Select disks & apply

28. Measurement of zone size

29. • Interpret the results by comparing zone size
with interpreting chart

30. Automated zone readers

31. Advantage
• It is very convenient,qualitatively efficient and is
inexpensive.
Disadvantage
• High chances of contamination.
• MIC can’t be determined.
• Proper technique required otherwise zones
are not properly formed.
• Proper distance between 2 disks is required
otherwise zone will overlap each other

32. • It is same as disk diffusion
• Single step process
• Equal distance between two
disk

34. • A plastic strip having
gradually decreasing
concentration of a
particular antibiotic.
• It also determines MIC

35. Interpretation

37. • Detect the presence of a particular resistance
mechanism.
• BY genotypic methods we can test for the
specific genes which are responsible for
antibiotic resistance.
• PCR & DNA hybridization are used

38. 1) pH of Medium
eg.streptomycin is more effective at
alkaline pH
2)Depth of medium
3)Antimicrobial agent
a) diffusion ability
b) molecular weight

39. 4) Inoculum size
5) Disk

40. Thank you